Explosion detection and suppression



y 1957, B. 'r. JOYCE I ETAL' 2,79 ,7

EXPLOSION DETECTION AND SUPPRESSION Filed June 25, 1953 0.0. POWERSUPPLY +I5O VOLTS l6 EXPLOSION SUPPRESSOR PHOTOCONDUCTIVE {5 PHOTOCELL42 EXPLOSION DIFFERENTIATING SUPPRESSOR CIRCUIT \35 FIG. 2

EXPLOSION SUPPRESSOR 2 FUEL & TANK g 49 p v, INVENTORS B.T..JOYCE 1A..F.KRUEGER ATTORNEY United States Patent EXPLOSION DETECTION ANDSUPPRESSION Bradford T. Joyce, Brookline, and Albert F. Krueger,Needham, Mass., assignors to Electronics Corporation of America, acorporation of Massachusetts Application June 25, 1953, Serial No.364,142

3 Claims. (Cl. 250-214) This invention relates to apparatus to detect anincipient explosion and to suppress it before it has reached a dangerousenergylcvel. More particularly, it relates to the detection andsuppression of explosions occuring in mixtures of air with combustiblevapors or gases, sprays, mists, dusts or the like, and with theprevention or extinction of fires resulting from such explosions.

As compared with the explosion of a substance which carries its ownoxygen, an explosion of a mixture such as that of air and gasoline hasthe characteristic that the rate of development of presure is relativelyslow; thus, for example, an explosion of a mixture of air and parafl'inspray may take 0.5 second to develop maximum pressure. a

Thus an explosion of a mixture of a combustible vapor with air beginswith a relatively slow build-up of energy, with correspondingly slowrises of pressure, heat and radiation. This rise may be spread overseveral hundredths of a second before a critical energy level is reachedand a dangerous explosive pressure is developed.

An explosion detector and suppressor system has been proposed in whichthe detector comprises a pressureresponsive diaphragm which detects therise in pressure occuring in an incipient explosion. This detector isconnected in an electric circuit including a contact adapted to beclosed by the detector to release an explosion or fire suppressingsubstance. The detector is made to respond to a small pressure rise whenthe rate of pressure rise is in excess of 50 lbs. per sq. in. persecond, and is capable of closing the contact within .02 second of theinitiation of the pressure rise. The suppressor is capable ofdistributing the explosion or fire suppressing substance within .05second of the initiation of the pressure rise. The suppressor comprisesa frangible container for the explosion or fire suppressing substance. Arapidly-acting detonator, which acts in response to the closing of thedetector contact, is arranged to burst the container and thus scatterthe explosion or fire suppressing substance which it contains. Thissystem is described in detail in British Patent No. 643,188, publishedSeptember 15, 1950.

The principal object of this invention is to provide an explosiondetector which, together with a suppressor of the kind described above,combines to make an improved explosion detector and suppressor systemwhich is faster acting and more sensitive than prior-art devices.

The explosion detector of this invention is essentially a device whichis sensitive to the rise of electromagnetic radiation which occurs in anincipient explosion. This electromagnetic radiation is a flash of lightdetectable by a photocell, the world light being used here in its widersense which includes light visible and invisible to the human eye. Themost suitable type of photocell for this application is a lead sulfidephotoconductive cell such as those shown in U. S. Patents 2,448,516 and2,636,100. It comprises a photocell, the output of which is connectedtoan electronic valve through a diflerentiating network. This valve isnormally nonconductive, and conductswhen the photocell detects a rise ofradiation of thetype found in incipient explosions. Conduction of thevalve actuates the explosion suppressor.

Other and incidental objects of the present invention' will be apparentto those skilled in the art from a reading of this specification and aninspection of the accompanying drawing,- in which: Figure 1 shows anembodiment of the present invention; Figure 2 shows another embodimentof the present invention; and Figure 3 shows schematically av fuel tankequipped with the explosion detector and suppressor of the presentinvention.

Referring to Figure 1, there is shown an explosion suppressor 3comprising a relatively stiff backing plate 5 which includes a housing 7for a detonator 9. The wall of the housing 7 is reduced in thickness atthe working 'end of the detonator 9. A frangible cup 11, made ofphenolic-impregnated fabric, is securely attached to the backing plate:5 and forms a sealed cavity 13 in which The explosion detector comprisesa photocell 15 which p,

is shown'as being of the photoconductive type. One terminal 16 of thephotocell 15 is connected to a D. C. power supply 17, and the otherterminal 18 of photocell 15 is connected to ground through a resistor19. The potential at terminal 16 is positive, and its magnitude may beof the order of volts. Terminal 18 is connected through adifferentiating circuit 21 and resistor 23 to the control grid 25 of anelectronic valve 27 which is shown as an electron tube of the thyratrontype. Differentiating circuit 21 comprises a capacitor 29 and a resistor31, one terminal of which is connected to the junction of capacitor 29and resistor 23, and the other terminal of which is connected to asource of biasing potential. This source of biasing potential is shownas a potentiometer 33 connected between a source of biasing potential 35and ground. The anode 36 of valve 27 is connected to power supply 17through resistor 37, and to ground through capacitor 39. The cathode 41of valve 27 is connected to ground through the detonator 9 whichincludes a low electrical resistance.

The operation of the explosion detector of Figure 1 is as follows: thephotocell 15 and resistor 19 form a potential divider connected betweenterminal 16 and ground. The potential at terminal 18 thus varies withthe resistance of photocell 15 and, consequently, with the amount ofradiant energy impinging upon photocell 15. A negative biasing voltagefrom potential source 35 is applied through resistors 31 and 23 to thecontrol grid 25 of thyratron 27 which is thus normally held in anonconductive state. Terminal 18 is coupled to the control grid 25 ofthyratron 2'7 through differentiating circuit 21. The time constant ofdifferentiating circuit 21 should be chosen so that it will pass apositive pulse of sufiicient magnitude to overcome the bias on thyratron27 and fire it only when the rate of change of potential at terminal 18corresponds to that which would be caused by an incipient explosion. Atime constant of 10 milliseconds has been found satisfactory for thedetection and suppression of explosions in a mixture of fuel with air.When the bias on thyratron 27 is overcome and it fires, capacitor 39,which was previously charged through resistor 37, discharges throughthyratron 27 to ground, and this discharge fires detonator 9 The firingof detonator 9 bursts the cup 11, and the explosion or fire suppressingfluid which it contains is scattered, thus suppressing the explosion.

The explosion detector of Figure 2 differs from that of Figure 1 in thatthe anode of thyratron 27 is connected,

Patented July 16, 1957 3 through the high-voltage winding 42 of atransformer 43, to an alternating current power supply of the typecustomarily found in aircraft, The low-voltage winding 47 of transformer43 is connected: to the detonator 9. The. advantage of this embodimentis that itavoids the use of the large electrolytic capacitor 39 ofFigure 1. As in Figure 1, thyratron 2,7, is kept in a nonconductivestate until the radiant energy from an incipient explosion impinges uponphotocell 15. Thyratron 27 then conducts, and the'current through,winding 47 of transformer 43 fires the detonator 9', thus operatingthe'suppressor 3.

The photoconductive cell is preferably of the activated' lead sulfidetype, 'which has good. sensitivity in the infrared portion of thefrequency spectrum. The explosion or fire suppressing fluid may bemethyl bromide, carbon tetrachloride or any other extinguishant. Waterhas been used successfully in. some tests; it acts mainly by.- cooling.Gasoline has also been used successfully; it can suppress an explosionby creating too rich a. mixture of fuel with air.

Figure 3 illustrates an aircraft fuel tank 49 equipped with an explosiondetector shown schematically at 51 and with an explosion suppressor 3.The source of ignition of an explosion is shown at 53, and the flamefront of the explosion is shown at 55'. The radiant energy from theincipient explosion travels with the speed of light and impinges uponthe detector 51. Detector 51 actuates the suppressor 3 which scattersthe explosion or fire sup pressing fluid as shown schematically by thebroken lines of Figure 3.

We claim:

1. An explosion detector comprising: a radiation-sensitive circuitincluding a photoelectric cell, an electron tube having an anode, acathode and a control electrode, biasing means to maintain said electrontube normally nonconductive, a capacitor connected between the anode ofsaid electron tube and a point of reference potential, means adapted tocharge said, capacitor, means to connect to the cathode of said electrontube explosion suppressor means responsive to the discharge of saidcapacitor through said electron tube, and means including adilferentiatifig c1rcuit connected between said radiation-sensitivecircuit and the control electrode of said electron tube.

2. An explosion detector comprising: a radiationsensitive circuitincluding a photoelectric cell, an electron tube having an anode, acathode and a control electrode, biasing means to maintain said electrontube normally nonconductive, a source of alternating current, atransformer having two windings, means to connect a first winding ofsaid transformer between the anode of said electron tube and saidsource, means to connect the cathode of said electron tube to a point ofreference potential, means to connect to a second winding of saidtransformer explosion suppressor means responsive to the passage ofcurrent through said transformer and through said electron tube, andmeans including a differentiating circuit connected between saidradiation-sensitive circuit and. the control electrode of said electrontube.

3.. An explosion detector and suppressor system comprising: aradiation-sensitive circuit including a photoconductive cell, anelectronic valve having an input and an output, biasing means tomaintain said electronic valve normally nonconduct-ive,detonator-actuated explosion suppressor means responsive to conductionthrough said valve, means to connect said explosion suppressor means tothe output of said valve, and means including a differentiating circuitconnected between said radiationsensitive circuit and the input of saidvalve.

References Cited in the file of this patent UNITED STATES PATENTS1,453,091 Delbare Apr. 24, 1923 1,708,869 Buddecke Apr. 9, 19292,355,664 McMann Aug. 15, 1944 2,492,148 Herbold Dec. 27, 1949 2,570,280Rotfman Oct. 9, 1951 2,668,289 Conrad et al Feb. 9, 1954

1. AN EXPLOSION DETECTOR COMPRISING: A RADIATION-SENSITIVE CIRCUITINCLUDING A PHOTOELECTRIC CELL, AN ELECTRON TUBE HAVING AN ANODE, ACATHODE AND A CONTROL ELECTRODE, BIASING MEANS TO MAINTAIN SAID ELECTRONTUBE NORMALLY NONCONDUCTIVE, A CAPACITOR CONNECTED BETWEEN THE ANODE OFSAID ELECTRON TUBE AND A POINT OF REFERENCE POTENTIAL, MEANS ADAPTED TOCHARGE SAID CAPACITOR, MEANS TO CONNECT TO THE CATHODE OF SAID ELECTRONTUBE, EXPLOSION SUPPRESSOR MEANS RESPONSIVE TO THE DISCHARGE OF SAIDCAPACITOR THROUGH SAID ELECTRON TUBE, AND MEANS INCLUDING ADIFFERENTIATING CIRCUIT CONNECTED BETWEEN SAID RADIATION-SENSITIVECIRCUIT AND THE CONTROL ELECTRODE OF SAID ELECTRON TUBE.